1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method.
2. Description of the Related Art
With a view to purifying exhaust gas containing air pollutants, such as HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxides), an exhaust gas purifying catalyst is provided, for example, in an engine exhaust system of a vehicle, such as an automobile. By use of such an exhaust gas purifying catalyst, HC/CO and NOx can be converted into inert emissions through an oxidation reaction and a reduction reaction, respectively.
As a component for use in the exhaust gas purifying catalyst, there has been known an oxygen-storage material which has a property (so called “oxygen storage/releasing capability”) capable of absorbing and storing oxygen in exhaust gas when an exhaust atmosphere is in an oxygen-excess state, i.e., an exhaust air/fuel (A/F) ratio is on a lean side relative to a theoretical (stoichiometric) exhaust A/F ratio, and releasing the oxygen stored therein when the exhaust atmosphere is in an oxygen-deficient state i.e., the exhaust A/F ratio is on a rich side relative to the theoretical exhaust A/F ratio.
For example, a composite oxide containing CeO2 or CeO2—ZrO2 is known as the oxygen-storage material. As one specific example, the Japanese Patent Laid-Open Publication No. 2004-174490 discloses a technique intended to obtain a composite oxide containing Ce, Zr, and Rh as a catalytic metal in an evenly dispersed state, so as to provide a catalyst material having enhanced catalytic conversion efficiency.
As another specific example, the Japanese Patent Laid-Open Publication No. 2002-282697 discloses an exhaust gas purifying catalyst intended to reduce an amount of HC to be discharged during a period before a catalytic metal carried on a catalyst layer is activated, without increasing a thickness of a HC absorbent layer, wherein the exhaust gas purifying catalyst has a three-way catalyst layer which comprises CeO2—ZrO2 and Rh/ZrO2 consisting of a carrier containing ZrO2 and Rh carried on the carrier.
In the conventional composite oxide containing Ce, Zr, and Rh as a catalytic metal, most of the Rh used as a catalytic metal exists within oxide particles, and only a part of the Rh is exposed from a surface of the composite oxide. Thus, as contrasted with an advantage of being able to actively absorb and release oxygen, the composite oxide has a problem about insufficiency of conversion efficiency for exhaust gas emissions absorbed on particles of the composite oxide, particularly in NOx (nitrogen oxides) conversion efficiency, due to limited opportunity of contact between the exhaust gas emissions and the Rh as a catalytic metal.
Currently, there is a strong need for providing an exhaust gas purifying catalyst having further enhanced conversion efficiency for not only NOx but also other exhaust gas emissions including HC and CO. Particular, in view of an engine cold-start condition causing difficulty in ensuring sufficient catalytic conversion efficiency due to a low catalyst temperature, a catalyst temperature is low, and it is required to improve low-temperature catalytic activity in the exhaust gas purifying catalyst.